1. Field of the Invention
The present invention relates to an electrical device in general and a signal device in particular. The invention permits the user to control the power supplied to and the signal emitted by an output component of the device such as a signal lamp by translating or rotating a movable element. A threaded cover or base of the housing of the device is easily employed as the movable element.
Description of Prior Art
Prior art incorporates a high current potentiometer to regulate the power supplied to an electrical output element. The high current potentiometer is a variable resistor which is connected in series with the electrical output element. The resistance of the potentiometer is adjusted by rotating the knob of the potentiometer. It is a design which is lacking in several areas.
First the potentiometer is prone to failure as its moving parts wear. Second, the potentiometer has a control knob which must penetrate the body of the electrical device and in so doing reduces the strength of the case. For electrical devices which must be watertight, the opening in the body for the control knob can also be a source of leakage. Third, the projecting control knob is vulnerable to breakage. Fourth, the potentiometer must handle all the electrical current of the electrical device and therefore it is expensive. Fifth, the potentiometer is electrically connected to the source of power it therefore creates a safety hazard.
Another prior art design incorporates a metal oxide semiconductor field effect transistor (MOSFET) with its source and drain terminals in series with an electrical output element such as a high wattage incandescent lamp. The gate voltage of the MOSFET is controlled by a low current potentiometer. The MOSFET is a variable resistor which changes the resistance between its source and drain terminals in response to an applied gate voltage. Since the potentiometer only regulates the gate voltage it is only required to control a very small amount of power. The MOSFET handles the larger power necessary to control the high wattage lamp. This design reduces the power controlled by the potentiometer and improves the reliability of the electrical device. However, it does not eliminate many of the previously described problems associated with the high current potentiometer design. The design can work acceptably well if due consideration is given to the design of the low current potentiometer and its interaction with the gate control circuit of the MOSFET. An acceptable limited rotation of the knob on the potentiometer must effect the change in gate voltage required by the MOSFET to alter its resistance sufficiently for the lamp to have its intensity sufficiently altered. If a small rotation of the potentiometer knob creates too large a change in gate voltage the lamp will change from "DIM" to "BRIGHT" too quickly and the user will not be able to adjust to the precise intensity needed. On the other hand, if complete rotation of the potentiometer knob does not affect an adequate change in gate voltage, the resistance of the MOSFET will remain high and the lamp will not achieve full intensity. Potentiometers can be designed to provide almost any range of resistance values and can therefore usually be designed to permit adjustment of the gate voltage within the required range. The MOSFET must also include an acceptable range of resistance. Since the MOSFET is in series with the lamp, its resistance at its low resistance value must be low enough relative to the lamps resistance to permit the lamp to become to be acceptably "BRIGHT" and its resistance at its high resistance value must be large enough relative to the lamps resistance to assure that the lamp becomes acceptably "DIM". Unfortunately, there are only a limited number of MOSFETs from which to choose each with different initial and final resistance values. Therefore, there is no assurance that an acceptable MOSFET is available for a particular lamp. Also each MOSFET changes its resistance at a rate of change which is related to the rate of change of its gate voltage. Therefore, the operator must have the ability to change the gate voltage at a rate of change that correlates with the required rate of change of the resistance of the MOSFET and in turn the required rate of change of the intensity of the lamp. Some embodiments require a linear change in gate voltage and for these the potentiometer can function very well. However, other embodiments require a non-linear change in gate voltage. In this regard, potentiometers which are limited in their ability to create a non-linear rate of change of their resistance are thus limited in their ability to change the gate voltage at a required non-linear rate of change to thereby change the intensity of the lamp at a required rate. If the potentiometer does not include the necessary rate of change of its resistance as the operator rotates the potentiometer knob, he will change the lamp intensity from "DIM" to "BRIGHT" but the rate of change of intensity will not be as desired. It is noteworthy to realize that a second resistive element must be placed in series with the potentiometer if it is to provide a variable gate voltage. This second resistive element adds expense and complexity to the device.